Various types of medical devices are employed to monitor electrical or other activity of the heart and to provide therapy to the heart for the correction of electrical conduction defects, inadequate pump function, or irregular cardiac rhythms. Such defects and irregular rhythms may result from or indicate various pathological conditions of the heart, including congestive heart failure. Many such devices are implantable beneath the skin of a patient, i.e., in the patient's chest. Such implantable devices include a hermetically sealed canister containing electronic circuitry for implementing the functions of the device, one or more electrodes implanted in one or more of the ventricles and/or atria of the heart, or in close proximity thereto, and leads for connecting the electrodes to the circuitry within the device canister. The device circuitry includes circuitry for detecting electrical signals produced by the heart, which signals are picked up at the electrodes, along with circuitry, typically implemented in a microprocessor, for analyzing the thus detected cardiac signals. The device may also include circuitry for providing therapy in the form of electrical signals applied to the heart. Such signals are provided to the heart via the leads and electrodes mounted in the heart so as to correct electrical conduction defects or abnormal rhythms. The analysis circuitry controls the delivery of such electrical therapy signals based on the detected cardiac activity signals. The implantable device may also include a transmitter/receiver for transmitting cardiac activity and other information to an external device for, e.g., storage and/or further analysis, and for receiving information, such as programming instructions, from the external device via, for example, an RF link.
An example of such an implantable cardiac device is an implantable cardiac pacemaker. A pacemaker provides relatively low-level electrical pulses to the heart to stimulate heart activity when the natural cardiac rate provided by the heart is too low. A dual chamber pacemaker includes electrodes positioned in both the atria and ventricles of the heart for detecting naturally occurring atrial and ventricular activations, and for providing pacing pulses to the atria and/or ventricles as needed. Such a device monitors the time between sensed and paced atrial and ventricular activations and provides pacing pulses as needed to maintain an adequate heart rate. For example, such a device will note the occurrence of a sensed or paced atrial or ventricular event and, if a subsequent naturally occurring atrial and/or ventricular event is not sensed within a certain time (escape interval) following the first sensed or paced event, a pacing pulse will be applied to the atria and/or ventricles to maintain a desired heart rate.
Implantable cardiac pacemakers may also be employed to improve cardiac function in heart failure patients. Such implantable pacemakers may improve cardiac pump function by correcting left ventricular conduction system defects. In such systems an electrical pacing signal is provided by the pacemaker to a pacing lead implanted epicardially in the coronary vein to stimulate the left ventricle. Biventricular pacing may also be provided. Such heart failure therapy is commonly referred to as cardiac resynchronization therapy. Implantable pacemakers for heart failure therapy may be implemented as dual or triple chamber pacemakers. Such pacemakers may be included within an implantable device capable also of providing atrial and/or ventricular defibrillation.
As discussed above, left ventricular or biventricular pacing can have a beneficial effect for patients suffering from congestive heart failure. However, it has been found that the various parameters which control such pacing must be set within a certain range for such pacing to have a beneficial effect for those patients, or to yield the best therapeutic response. For example, it has been found that patients exhibiting congestive heart failure have a specific range or value of AV delay (the maximum time allowed to pass following a sensed or paced atrial event before the ventricles are paced), over which the therapeutic response to pacing is positive. Also, within the range of AV delay, or other pacemaker settings, for which a positive therapeutic effect is provided by pacing, is a more narrow optimum subrange of AV delay or other pacing parameter values which provide the greatest degree of therapeutic improvement. Arbitrarily setting the AV delay and other pacing parameters at nominal values may not guarantee the optimum benefit obtainable (or even any benefit at all) by left ventricular or biventricular pacing in congestive heart failure patients. It would, therefore, be advantageous to have a system which allows an operator to determine whether a controlled pacing parameter or set of parameters is optimal for a given patient and, if not, which allows the operator to reprogram an implanted pacemaker such that the implanted device is made to operate in a pacing mode and with pacing parameters that result in an optimum benefit to the patient.
U.S. Pat. No. 5,540,727 to Tockman, et al. describes a method and apparatus for optimizing the pacing mode and pacing cycle parameters of an implanted dual chamber pacemaker. This patent describes providing as inputs to an external monitor/programming device data from one or more cardiac performance monitoring devices. Such devices may include external equipment, such as an acceleration sensor, capable of providing signals characteristic of features of the mechanical movements of the heart muscle, its valves, and the blood being pumped by it, a Doppler flow sensor for sensing cardiac output, a pressure cuff or similar type sensor for providing an indication of mean arterial pressure, a pulse oximeter to provide signals corresponding to the percentage concentration of oxygen and carbon dioxide in the patient's blood, and/or a respiration sensor capable of analyzing respiratory gases and delivering signals proportional thereto to the monitor/programmer. One or more sensors may also, or alternatively, be implanted within the body to sense various cardiac performance parameters and to provide corresponding data to the monitor/programmer. Data from the cardiac performance monitoring devices is used by the monitor/programmer to monitor one or more physiological parameters during a pacing interval with a given pacing parameter (e.g., AV delay) set at a given value and with a given pacing mode. The selected pacing parameter is changed incrementally, and monitoring of the detected physiological parameters is repeated at each increment of the pacing parameter. Between each such pacing period, the heart is allowed to beat at a natural sinus rhythm, in order to establish a baseline for comparison. The physiological parameters monitored during each pacing interval are analyzed to determine the selected pacing parameter value and pacing mode which results in the best resulting physiological response, which could be either a maximum or minimum value of a physiological parameter, depending upon the particular physiological parameter involved. An implanted pacemaker is then programmed using the external programmer to operate with the pacing parameter value and pacing mode associated with the best determined physiological parameter response.
A known method of evaluating heart mechanical performance is described in U.S. Pat. No. 5,291,895 to McIntyre. This patent describes a system for evaluating cardiac performance which employs a finger pressure sensor including a piezoelectric pulse pickup and an inflatable cuff which wraps around a patient's finger to apply pressure, through the pickup, to the skin of the patient's finger. Pressure pulses detected using the finger sensor may be analyzed to detect abnormal cardiac performance. In particular, abnormal cardiac performance may be identified by analyzing the finger pressure signal obtained during and after an activity which produces stress on the patient's heart. An example of such a heart-straining activity is the Valsalva maneuver, in which a patient exhales forcefully into a closed system so as to maintain an increased intrathoracic pressure for a short period of time. It has been determined that the reflected arterial-pulse contour detected from a patient's finger during the Valsalva maneuver is predictive of pulmonary-capillary wedge pressure. In particular, the pulse-amplitude ratio of the final (minimal) to initial (maximal) amplitude of the strain phase of the maneuver has been determined to correlate well to pulmonary-capillary wedge pressure. Pulmonary-capillary wedge pressure is a well-known measure of heart failure disease.
What is desired is a system and method which allows a physician to easily, accurately, and noninvasively monitor the condition of a heart failure patient and to optimize the pacing parameters of an implanted pacemaker used to treat such a condition. In particular, what is desired is a system and method which allows a physician to monitor the condition of a heart failure patient and to optimize the pacing parameters in an implanted pacemaker providing therapy to the patient by noninvasively monitoring a physiological parameter related to heart performance and programming the implanted device to optimize such a parameter.